Electrostatic separator carrier electrode



Dec. 12, 1961 F. FRAAS ELECTROSTATIC SEPARATOR CARRIER ELECTRODE Filed Dec. 8, 1959 INVENTOR FOSTER FRAAS BY W5 Maw ATTORNEYS United States Patent" 3,012,668 ELECTROSTATIC SEPARATOR CARRIER ELECTRODE Foster Fraas, Hyattsville, Md., assignor to the United States of America as represented by the Secretary of the Interior Filed Dec. 8, 1959, Ser. No. 858,002 Claims priority, application Germany Sept. 11, 1959 7 Claims. (Cl. 209-127) (Granted under Title 35, U.S. Code (1952), see. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royal-ties therein or therefor.

This invention relates to the electrostatic separation of minerals or other particulate material and more particularly to improved electrode means in electrostatic separating devices.

Electrostatic separator, other than the free fall type, have one or more electrodes in contact with the material being separated. Since these electrodes generally serve to transport the material as well, they are designated as carrier or transport electrodes. They may take the form of rotating cylinders or rollers, moving belts, or inclined plates, according to the type or design of electrostatic separator in which they are employed. All of these modifications are conventional, and are well known to the art. The description of the invention will be confined solely to the rotating cylinder-type electrode for the sake of simplicity, but it is to be understood that analogous results are obtained as well with the other forms.

Previously, such transport electrodes have been constructed of highest conductivity conductors, i.e. metals. I have now discovered that improved results may be obtained with nonmetals of special electrical properties on the transport roller.

One object of this invention is to provide a more efficient separation of granular material by having an intrinsic or lattice defect type semiconductor for the surface of the conveying electrode which contacts the granular material.

Another object of this invention is to provide an improved electrostatic separator having a solid high resistance film on the conductive conveying electrode which contacts the granular material.

A further object of this invention is to provide a contacting electrode having a high resistance metal oxide surface.

Further objects of this invention will appear from the following description taken in connection with the draw- In the figure, which is a schematic representation of a roll-type electrostatic separator, the material which is to be separated is stored in hopper 1 and passes over feeding belt 2 to rotating cylinder 3, which is the carrier electrode. Spaced near cylinder 3 is electrode 4, which may have a small, or a relatively large surface.

The diameter of electrode 4 determines the nature of the field. If this electrode has a very small diameter and concomitantly small surface, the electrical field intensity at its surface is sufficiently high to ionize the surrounding atmosphere, and the space between electrodes 3 and 4 is ionized. If electrode 4 has a relatively large diameter, as for example one inch or more, there is no ionization, and an electrostatic field is produced.

Roller or cylinder 3, may be constructed of an electrically conductive or non-conductive material, depending upon the material constituting the coating or film 5, thereon. A' high resistance oxide film requires that the base be of an electrically conductive material such as metals,

"ice

' a base of electrically conducting materials such as metals.

In one embodiment, as will be described below, the cylinder 3 may be aluminum covered with a high resistance oxide film 5. The oxide film may be aluminum oxide deposited electrolytically on the roll surface. Conductors 6 and 7 join electrodes 4 to a DO. source 8 which is connected to a reversing switch 9 so that the polarity of the electrodes 3 and 4 may be reversed. Microammeter 10 and kilovoltmeter 11 measure the current and potential between the electrodes.

Under the influence of electrode 4, the material on bearing roller 3 is diverted toward one side or the other of dividing blades 12 and 13. When an ionized discharge electrode is employed, blade 13 is in use. With an electrostatic field, blade 12, which as shown in the figure is spaced further from the contacting roll, is employed. Since the constituents of the material treated have different electrical properties, collection of fractions falling on different sides of the dividing blade results in separation or enrichment of the components.

In the prior art devices, the carrier electrodes were all made of high conductivity materials, e.g. metals, probably on the belief that greatest selectivity would result with the most conductive contact surface. I have found that selectivity may be improved by coating the roll with a high resistance solid film, or with a semiconducting lay-- er. The reasons for the improved results are not fully understood at present. It is believed, however, that the analogy with a semiconductor rectifier offers some hint as to the explanation. Most minerals are semiconductors. Improved rectifying action has been shown to occur when a high resistance layer is interposed between a semiconductor and a metal. and Gurney in Electronic Processes in Ionic Crystals, Oxford Press, 1940, p. 178. It would appear that at the carrier electrode-particle contact the directional effect of charge transfer representative of the semiconductor particle is not obtained unless the resistivity is sufficiently high.

Another explanation, applicable to the semiconductinglayer on the roll modification, is the Johnsen-Rahbek effect which results when a semiconductor is brought into contact with a conductor and ,a potential dillerence is applied between the two. The result is a force which attracts the two together. This is discussed by Zwikker in Physical Properties of Solid Materials, Pergamon Press,

1955, p. 258. In the adaptation of-this effect to electrostatic separation of particulate material, the potential difference between a conductor particle and a semiconductor conveying electrode can only be obtained from the particle charge produced by the ionized discharge from a spaced ionizing electrode. field electrode does not produce such a potential difference. trode does occur but it is of the same polarity as the conveying electrode resulting in a repulsion instead of an attraction.

In the following examples comparison separation tests were made with conveying electrodes having semicon-' ducting surfaces, and conveying electrodes of aluminum The charge and electrostatic field.

This is discussed by Mott.

In contrast, a static' Charging of the particle with a static field elec-f between the electrodes contained positive or negative ions, depending on the polarity of the high potential. Current fiow was measured with the microammeter 10. For creating a static electrical field, a one-inch diameter cylinder replaced the high potential wire at 4. Here, the potential difference was measured by the kilovoltmeter 11.

For test purposes only half the length of the carrier rolls were provided with the improved surfaces, the remaining half length was of metal for comparison purposes. It is understood, of course, that in actual use for mineral separation no metal surface is retained. The improved surfaces are shown by the following illustrations:

A. Aluminum oxide.-Half of an aluminum cylindrical carrier electrode was anodized by electrolytic treatment as an anode in 10% sulfuric acid, washed in dilute ammonia, and dried.

B. Nickel xide.A nickel cylindrical carrier electrode was oxidized by heating in air in the increasing temperature range of 900 to 1000 C. for 30 minutes. Half of the length of oxide covering on the cylinder was then removed by abrading with emery cloth.

C. Copper 0xide.A refractory aluminum oxide carrier electrode was coated by painting with a water suspension of cuprous oxide, dried and heated in air to 1100 C. After cooling, a hard adherent surface of black cupric oxide is obtained which passes around the ends of the electrode to electrically contact the grounded shaft. The cupric oxide coating is a semiconductor and much thicker than the nickel and aluminum oxide films. For comparison with an aluminum metal surface half of the length of the electrode was covered with aluminum foil.

In the tests, each sample was fed successively over the metal surfaced portion of the cylinder and then over the improved surfaces. Dividing blade 13 (or 12) was spaced approximately 0.15 inch from the roll surface and split the feed into conducting (nomadhering) and non-conducting (adherin particles. That portion collected which was furthest from the roll represented the conducting fraction. By correlating the amount of conductor fraction in a mineral mixture, as a percent of the total sample, with the magnitude of the current or potential, the elfect of the carrier electrode surface which was employed was determined.

EXAMPLE 1 Variation of the conductor fraction separated with (a) polarity and (b) carrier electrode surface is shown in the following Table I. Ion discharge was employed.

This shows the increase in response obtained with the improved surfaces. Comparison of the test results of magnitite, an n-type semiconductor and galena, a p-type, illustrate the effect of polarity on semi-conductor type minerals.

4 EXAMPLE 2 A mixture of 46% galena and 54% magnetite was passed through a separator having an aluminum metal EXAMPLE 3 The efiect of an improved roll is shown in this experimerit.

A mixture of 48% galena and 52% magnetite was passed through a separator having an aluminum oxide coated aluminum roller at 20 microamperes current. The nonconductor portion recovered from the first pass was repassed to illustrate the high efficiency obtainable with multiple passes.

Table III Coni- Percent Fraction Weight, position, of Total Percent. Percent Galen-e. Galena 1st pass, conductor 27 99 56 2nd pass, conductor. 10 83 17 Noncouductor 63 20 27 Total 100 48 100 EXAMPLE 4 Similar tests to the above were carried out with mixtures of columbite and ilmenite using the same type rolls and repassing the non-conductor to fraction twice to provide the equivalent of a three roll separator.

Table IV Separation of columbite from ilmenite using a metal what 150 microamperes.

Corn- Percent Fraction Weight, position, of Total Percent Percent Columbitc columbite 1st Conductor 18 35 12 2nd Conductor 13 36 9 3rd Conductor..- 9 35 6 Nonconductor 60 G5 7 Total 100 53 100 Table V Separation of columbite from ilmenite with an oxide roll and 10 microamperes current.

Oom- Percent Fraction Weight, position, of Total Percent Percent columbite Columhite 1st Conductor 25 44 2nd Oon luct0r.- 11 76 17 3rd Conductor..- 10 G3 12 N onconductor 54 25 27 Total 51 100 EXAMPLE 5 In the above Examples 1-4, an ionizing electrode was employed. Similar improved results are obtained with a coated roll in electrostatic field separation. A synthetic black nickel oxide containing percent lithium oxide, a known p-type semiconductor, was employed to show the effects of base aluminum metal and coated aluminum metal rolls in electrostatic separation.

Table VI Eflect of roll surface with an adjacent static field electrode.

Other metals and oxide coatings may be employed in the process, the only criterion being that the oxide coatings be electrically of high resistance. The metal of the metal oxide need not be the same as the metal of the roll. For example, a steel roll coated with an aluminum oxide layer may be employed. Solid insulating films other than metal oxide may be used, although usually these latter will prove to be most suitable.

The semiconductor layer may have a base other than the aluminum oxide of the embodiment described under illustration C above. Other ceramic or refractory bases may be employed, such as porcelain, silicon carbide, quartz, etc. Instead of copper oxide, semiconductors such as silicon, ferric oxide and lead sulfide for example, may be substituted.

In some instances the semiconductor layer may be formed in situ on a conducting roller by reacting the surface thereof with the proper substance. Examples of this modification are a semiconducting layer of AlSb on an aluminum roller formed by reacting antimony on the surface of the roller, and semiconductor titanium oxide formed by reacting oxygen with the surface metal of a titanium roller. Either intrinsic or defect type semiconductor surfaces may be employed to good eifect.

While particular embodiments of the present invention have been disclosed and described, it is to be understood that I am not to be limited thereby, and only such limitations would be imposed as are indicated in the appended claims.

I claim:

1. In an electrostatic separator for particulate material having at least one pair of spaced electrically conducting electrode means of opposite polarity, one of said pair of electrode means being cylindrical and rotatable and adapted to contact said material, wherein a portion of the material is deflected from the normal trajectory to a greater degree than the remainder whereby separation may be efiected, the improvement which comprises providing said contacting electrode means with a metal base portion having an electrically high resistance metal oxide layer as a thin coating thereon.

2. The improvement as in claim 1, wherein the base portion of the contacting electrode means is aluminum and the high resistance layer is aluminum oxide.

3. The improvement as in claim 1, wherein the base portion of the contacting electrode means is nickel and the high resistance layer is nickel oxide.

4. In an electrostatic separator for particulate material having at least one pair of spaced electrically conducting electrode means of opposite polarity adapted to produce a unidirectional ionized field, one member of said one pair of electrode means being cylindrical and rotatable and adapted to contact said material, wherein a portion of the material is deflected from its normal course by the ionized field, so that separation may be effected, the improvement wherein said one cylindrical and rotatable electrode means consists of a base portion composed of a material which is a member selected from the class consisting of metals and refractories, and coated with a thin semiconductor layer.

5. In an electrostatic separator for particulate material having at least one pair of spaced electrically conducting electrodes of opposite polarity adapted to produce a unidirectional ionized field, one member of said one. pair of electrodes being cylindrical and rotatable and adapted to contact said material, wherein a portion of the; material is deflected from its normal course by the ionized field, so that separation may be efiected, the improvement which comprises providing said contacting electrode with a thin semiconductor surface consisting of copper oxide supported on a ceramic base.

6. In an electrostatic separator for particulate material having at least one pair of spaced electrically conducting electrodes of opposite polarity adapted to produce a unidirectional ionized field, one member of said one pair of electrodes being cylindrical and rotatable and adapted to contact said material, wherein a portion of the material is deflected from its normal course by the ionized field, so that separation may be eiiected, the improvement wherein said contacting electrode is aluminum coated with a thin semiconductor layer consisting of AlSb.

7. In an electrostatic separator for particulate material having at least one pair of spaced electrically conducting electrodes of opposite polarity adapted to pro duce a unidirectional ionized field, one member of said one pair of electrodes being cylindrical and rotatable and adapted to contact said material, wherein a portion of the material is deflected from its normal course by the ionized field, so that separation may be effected, the improvement wherein said contacting electrode is titanium coated with a thin semiconductor layer consisting of titanium oxide.

References Cited in the file of this patent United States Bureau of Mines, RI. 3473, October 1939, pages 32-34. 

